Why does one breast suddenly hurt during breastfeeding? Breast pain during breastfeeding is a common occurrence, and there are several reasons for this: First, the ducts in the breast are not draining smoothly, causing milk to accumulate inside and pressing on the surrounding tissues, causing swelling and pain, sometimes on one side, sometimes on both sides, depending on the situation. Secondly, if there is a bacterial infection on top of the poor milk flow in the breast, if the bacteria multiply inside and cause mastitis, it also manifests as pain, not only pain but also other conditions such as redness, swelling, heat, fever, and if the blood test leukocytes will increase, this pain should be dealt with in a timely manner at the hospital for anti-inflammatory and other treatments. If the pain is only caused by poor milk flow, physiotherapy, massage, physical therapy to unblock the milk ducts, so that the milk ducts are smooth milk drainage, the pain will be reduced or alleviated, so whether one breast or both breasts pain, most of these causes.